1. Field of the Invention
This invention relates to a distance measuring device and more particularly to a so-called active type distance measuring device arranged to project a distance measuring light on an object and to receive a reflected light from the object.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings schematically shows an active-type distance measuring device of the prior art disclosed in Japanese Laid-Open Patent Application No. SHO 54-113356. The device of FIG. 1 includes a light emitting element ED1 which projects the light; a photo-electric converting element PD1 for receiving light; lenses LS1 and LS2 of a view finder optical system a half-mirror MR1; a total reflection mirror MR2; a frame FM for indicating a finder visual field and a distance measurement part; the eye EY of the photographer; and a light projecting lens LS3 disposed in front of the light emitting element ED1. The light from the light emitting element ED1 passes through the lens LS3 and is then reflected by a turning mirror MR4 for projection onto and scanning an object. Reflected light from an object is condensed by a lens LS4. The condensed light thus reflected strikes a light receiving or photo-sensitive thus reflected comes to a light receiving or photo-sensitive element PD1.
A photo-taking lens system LS5 is disposed within a lens barrel in a known manner. The lens system LS5 is shiftable back and forth together with the lens barrel along its optical axis. FIG. 1 further shows a diaphragm AD; a shutter SH; a film FL; a spring SP1 urging the lens barrel to move inwardly; a rack LG; a pinion G1 arranged to engage the rack LG; a ratchet G2 which rotates in one unified body with the pinion G1; a magnet Mg1 arranged to be controlled by a circuit which will be described later herein; an armature lever AM which is pivotally carried to be attracted by the magnet Mg1 and is provided with a stop pawl AM1 arranged to engage the teeth of the ratchet wheel G2; a spring SP2 arranged to urge the stop pawl SP2 to engage the ratchet wheel G2; an object Ob1 which is to be photographed and is located at a short distance; and another object Ob2 which is to be photographed and is located at a long distance. FIG. 2 is a graph showing signals produced from the photo-electric converting element PD1. The abscissa shows time (t) and the ordinate axis output voltage (V). Curve Q1 represents an output produced by a reflected light coming from the short distance object Ob1. Curve Q2 represents an output produced by a reflected light coming from the long distance object Ob2. The mirror MR4 turns in response to the movement of the lens barrel to scan objects with the light emitted from the light emitting element ED1 in the direction from the nearest distance point towards an infinite distance point. The photo-electric converting element PD1 receives reflected light from the object. When a peak value of signals produced from the element PD1 is detected, magnet Mg1 turns off. The spring SP2 pulls the stop pawl AM1 of the armature lever AM to engage the ratchet wheel G2. With the pawl engaged with the ratchet wheel, movement of the lens barrel is stopped by the pinion G1 and the rack LG to fix the position of the photo-taking lens LS5.
Measuring distances in the above-stated manner by projecting a distance measuring light flux of an approximately circular sectional shape projected onto an object to be photographed, has produced the following problem:
Where the object to be photographed is two persons 2a and 2b located somewhat away from each other as shown in FIG. 3, the scanning distance measuring light flux passes through a space between the two persons while the light flux is moving from a point 1b to another point 1a. Under such a condition, it is impossible to accurately measure a distance to the object. This problem has necessitated an additional distance measuring process. The camera is set once to have one of the two persons located in the center of the picture frame. The camera retains the distance information thus obtained and the camera is then "reset" to a position which will achieve the actually desired picture composition. Thus, it has been necessary to carry out a series of photographic processes.